1. Field of the Invention
The invention relates to an image forming apparatus, and particularly to an image forming apparatus designed such that a sheet having an image formed on a first side thereof by an image forming portion is again transported to the image forming portion so that an image may be formed on an opposite or second side of the sheet.
2. Related Background Art
Some of image forming apparatus such as copying machines, printers and facsimile apparatuses have heretofore been designed such that for example, a sheet having an image formed on a first side thereof by an image forming portion is again transported to the image forming portion so that an image may be formed on a second side of the sheet.
FIG. 16 of the accompanying drawings schematically shows the construction of a copying machine which is an example of such conventional image forming apparatuses, and in FIG. 16, the reference numeral 100 designates a copying machine, and the reference character 100A denotes a copying machine main body (hereinafter referred to as the apparatus main body), and a scanner portion 30 which is an image reading portion is disposed in the upper portion of the apparatus main body 100A, and an image forming portion 51 for forming an image on a sheet P is disposed in the central portion of the apparatus main body, and further, a cassette feeding portion 16 and a multi-feeding portion 40 which are a sheet feeding portion for feeding the sheet P to the image forming portion 51 are disposed in the lower portion of the apparatus main body.
Further, on a side portion of the apparatus main body 100A, there is provided a duplex transport portion 60 for again feeding the sheet P to the image forming portion 51 to form an image on the back side (second side) of the sheet P after an image has been formed on one side (first side) of the sheet P so that images can be formed on the two sides of the sheet P.
The image forming portion 51 has an electrophotographic photosensitive drum (hereinafter referred to as the photosensitive drum) 52, a developing device 53, etc. Design is made such that when a laser beam corresponding to image information emitted from a laser scanner 54 is scanned on the surface of the photosensitive drum 52, a latent image is formed on the surface of the photosensitive drum, and this latent image is developed by the developing device 53, whereby a toner image is formed on the surface of the photosensitive drum.
Design is also made such that when thereafter, the sheet is transported to a transferring portion constituted by the photosensitive drum 52 and a transfer charging device 55 as will be described later, the toner image formed on the photosensitive drum 52 is transferred to the sheet.
On the other hand, the cassette feeding portion 16 is for feeding the sheet to the transferring portion, and is provided with a sheet cassette 17 loaded on the bottom of the apparatus main body 100A, and a feeding roller 18 for feeding the sheets P contained in the sheet cassette 17. Design is made such that during image forming, the feeding roller 18 and a pair of retard rollers 19 are rotated in conformity with an image forming operation so as to separate and feed the sheets P one by one from the sheet cassette 17.
The multi-feeding portion 40 is also for feeding the sheet P, and is provided with a tray 41 openably and closably provided in the apparatus main body 100A, and a feeding roller 42 for feeding the sheets P stacked on the tray 41. Design is made such that during image forming, the feeding roller 42 is rotated in conformity with the image forming operation so as to feed the sheets P one by one from the tray 41.
On the other hand, the scanner portion 30 is for reading the image of an original placed on an original glass stand 301, and is designed to convert image information into an electrical image signal after it has read the image of the original, and input the image information converted into this electrical image signal to the laser scanner 54 of the already described image forming portion 51.
Description will now be made of the image forming operation of the copying machine 100 constructed as described above.
First, when the image information of the original is red by the scanner portion 30, this image information is image-processed, whereafter it is converted into an electrical signal and transmitted to the laser scanner 54 of the image forming portion 51. In some cases, the image information is inputted from an external device such as a personal computer (not shown) to the image forming portion 51.
In the image forming portion 51, the surface of the photosensitive drum 52 is scanned by a laser beam corresponding to the image information emitted from the laser scanner 54 to thereby form a latent image on the photosensitive drum, whereafter this latent image is developed by the developing device 53, whereby a toner image is formed on the surface of the photosensitive drum 52.
On the other hand, when in parallel to this operation, the sheets P are to be fed from the multi-feeding portion 40 to the image forming portion 51, the sheets P are fed one by one from the tray by the feeding roller 42. Also, when the sheets P are to be fed from the cassette feeding portion 16, the sheets P contained in the sheet 17 cassette are separated and fed one by one by the feeding roller 18 and the pair of retard rollers 19.
The sheet P thus fed from the multi-feeding portion 40 or the cassette feeding portion 16 is transported to a pair of registration rollers 1. At this time, the pair of registration rollers 1 which are registration means are at a halt, whereby the sheet P has its skew feed corrected, and is once stopped at this position and stands by. Thereafter, the pair of registration rollers 1 are rotated and the sheet P which has stood by is fed to the transferring portion constituted by the photosensitive drum 52 and the transfer charging device 55 in timed relationship therewith, and the toner image on the photosensitive drum is transferred to the sheet P when the sheet P passes through the nip part of the transferring portion.
The sheet P to which the toner image has been transferred in this manner is transported to a fixing apparatus 56, and is heated and pressurized when it passes through the fixing apparatus 56, whereby the toner image is fixed on the surface of the sheet. The sheet P on which the toner image has been thus fixed is thereafter delivered to a delivery tray 58 by the forward rotation of a sheet delivery roller 57 rotatable in forward and reverse directions.
Design is made such that when two-side printing is to be effected, a flapper 61 is changed over and the delivery roller 57 is rotated in the reverse direction after the trailing edge of the sheet delivered by the forward rotation of the delivery roller 57 has passed the flapper 61. Thereby, the sheet P comes into the re-transport path 62 of a duplex transport portion whereafter it is again transported to the image forming portion 51 by a duplex roller 63. An image for a second side is formed in this image forming portion 51, whereafter the sheet P is stacked on the delivery tray 58 by the delivery roller 57.
Now, the sheet P fed from the multi-feeding portion 40 or the cassette feeding portion 16 as already described has its leading edge rammed against the pair of registration rollers 1 being at a halt, and is thereafter further transported by a predetermined amount and forms a loop, whereby the skew feed thereof is corrected.
After the so-called registration correction by such a pair of registration rollers 1 has been effected, the pair of registration rollers 1 are rotated at predetermined timing to thereby transport the sheet P to the image forming portion side, whereby the toner image formed on the photosensitive drum can be transferred to a predetermined position on the sheet P.
FIG. 17 of the accompanying drawings illustrated the construction of the vicinity of the pair of registration rollers of such a copying machine 100, and in FIG. 17, the letter S designates a loop forming space provided downstream of the pair of registration rollers. Also, a first entry route R1 is provided between the cassette feeding portion 16 and the loop forming space S, and causes the sheet P fed from the cassette feeding portion 16 to enter the loop forming space S.
A second entry route R2 is provided between the multi-feeding portion 40 and the loop forming space S, and causes a sheet P1 fed from the multi-feeding portion 40 to enter the loop forming space S. The joining portion J between the first entry route R1 and the second entry route R2 which are fed sheet entry routes is located at the entrance of the loop forming space S. A relay roller 2 is provided in the first entry route R1.
As shown in FIG. 17, design is made such that the sheet P fed from the cassette feeding portion 16 travels toward the pair of registration rollers 1 via the first entry route R1 and the loop forming space S, and the sheet P fed from the multi-feeding portion 41 travels toward the pair of registration rollers 1 via the second entry route R2, the joining portion J and the loop forming space S.
Also, in FIG. 17, a third entry route R3 is provided between the re-transport path 62 and the loop forming space S, and is a curved re-transported sheet entry route for causing a sheet P2 which has passed the re-transport path 62 to enter the loop forming space S. The third entry route R3 is designed to join at the joining portion J between the first entry route R1 and the second entry route R2.
Thereby, the re-transported sheet P2 on one side of which an image has been formed and which has entered the re-transport path 62, and thereafter again travels toward the image forming portion 51 also passes the third entry route R3, and thereafter joins at the joining portion J between the first entry route R1 and the second entry route R2, and thereafter travels toward the pair of registration rollers 1 via the loop forming space S. After the re-transported sheet P2 has thus joined at the joining portion J, the re-transported sheet P2 can be made to abut against the pair of registration rollers 1 to thereby correct the skew feed thereof.
Now, when in such a conventional copying machine (image forming apparatus), the curved third entry route R3 of the re-transport path 62 is made to join the joining portion J between the first entry route R1 and the second entry route R2, a space for disposing the third entry route R3 therein becomes necessary sideways of the joining portion J, as shown in FIG. 17.
When the space for disposing the third entry route R3 therein is thus provided sideways of the joining portion J, the width of the apparatus main body 100A becomes great. Also, when as shown in FIG. 17, the multi-feeding portion 40 is disposed sideways of the re-transport route 62, the width of the apparatus main body 100A becomes still greater.
When the width of the apparatus main body 100A becomes greater as described above, a space necessary to install the apparatus main body 100A becomes wide and also, the apparatus main body 100A becomes bulky, and this leads to an increase in cost.